Mg-based alloy plated steel material

ABSTRACT

An Mg-based alloy plated steel material superior in adhesion and corrosion resistance characterized by being provided with a hot dip Mg-based alloy plating layer (preferably containing Zn: 15 atm % to less than 45 atm %).

This application is a national stage application of InternationalApplication No. PCT/JP2008/055189, filed 14 Mar. 2008, which claimspriority to Japanese Application Nos. 2007-066740, filed 15 Mar. 2007;and 2007-242561, filed 19 Sep. 2007, each of which is incorporated byreference in its entirety.

TECHNICAL FIELD

The present invention relates to an Mg-based alloy plated steel materialprovided with a high Mg composition alloy (Mg-based alloy).

BACKGROUND ART

As a hot dip metal plated steel material, a hot dip Zn plated steelmaterial is being used in a wide range of fields such as automobiles,building materials, household electrical appliances, etc. In general, ahigh amount of deposition of plating is effective for the purpose ofsecuring a long-term rust-proofing effect.

This is because with a Zn plating, the rate of corrosion of the platinglayer itself is slower than that of the iron metal of the steel materialand even at locations where the iron metal is exposed, the low corrosionpotential Zn exhibits a sacrificial corrosion-proofing ability withrespect to the steel material.

These corrosion resistant and corrosion-proofing effects are obtained bythe consumption of the Zn, so the greater the amount of Zn per unitarea, the longer the time the corrosion resistant and corrosion-proofingeffect can be maintained.

On the other hand, if the amount of deposition of Zn becomes greater,the workability, weldability, and other characteristics required for asteel material tend to deteriorate. For this reason, in Zn plating, ifpossible, exhibition of a high corrosion resistance by a smaller amountof deposition is being sought.

Further, in recent years, the depletion of Zn resources has beenconsidered a problem. To reduce the amount of use of Zn, Zn platinghaving a high corrosion resistance by a low amount of deposition isbeing sought.

To obtain a sufficient corrosion resistance by a low amount ofdeposition of Zn plating, an alloy element is added to the Zn plating toimprove the corrosion resistance. Up until now, numerous attempts havebeen made. In actuality, Zn—Ni alloy platings, Zn—Fe alloy platings,etc. are being widely used particularly for automobile steel sheet.Zn—Al alloy platings are also being widely used most for buildingmaterials.

In particular, to further improve the corrosion resistance in Zn—Alalloy platings, methods of adding Mg or Si are being developed. Forexample, the alloy plating layer of the steel superior in corrosionresistance disclosed in Japanese Patent Publication (A) No. 2002-60978contains, by mass %, Al: 1 to 50% and Mg: 0.1 to 20%.

Further, in the Zn—Mg alloy plating disclosed in Japanese PatentPublication (A) No. 2005-82834, the alloy plating layer contains, bymass %, 0.05 to 3% of Mg, whereby corrosion resistance is obtained. Inthis prior art, the Mg content of the plating layer is at most, by mass%, 20% or so.

In this way, in the prior art, the content of Mg has been kept low.There are mainly three reasons for this:

The first reason is that if adding Mg in a high concentration, thepossibility of rising the melting point of the plating bath rises andthat even after plating, the possibility of formation of intermetalliccompounds causing deterioration of the workability rises.

When adding Mg to the Zn bath, the Mg can relatively easily dissolvethere up to, by mass %, 3% or so. This is because the added Mg formsMgZn₂ (intermetallic compound) and this MgZn₂ forms a eutecticcomposition with Zn and causes the melting point to drop.

However, if adding Mg over 3%, the amount of formation of MgZn₂increases and result deviates from a eutectic composition, so themelting point of the plating bath rapidly rises and the viscosity of theplating bath rises.

Furthermore, if the amount of addition of Mg becomes close to 20%, theadded Mg forms insolubles and the amount of dross produced increases.The Mg accumulates in the dross at the plating bath surface in a highconcentration. Depending on the atmosphere, this ignites at the bathsurface. Plating becomes difficult.

Further, if adding Mg in a high concentration of 10% or more,intermetallic compounds and an alloy layer are formed in large amountsin the alloy plating layer after solidification.

The intermetallic compounds present in the alloy plating layer and thealloy layer formed at the boundary of the steel sheet and plating layerare poor in plastic deformability, so if using a plating bathcomposition containing Mg in a high concentration, a plating layer poorin workability is formed and the problems of cracking of the platinglayer and peeling from the steel sheet become remarkable.

Due to the above conditions enabling formation of plating and theproblem of workability of the plating layer, up until now the amount ofMg added has been considered limited to around a mass % of around 20%.

The second reason why the Mg content has been kept low is that Mg ispoor in reactivity with Fe. Mg does not form intermetallic compoundswith Fe and does not dissolve Fe at all (for example, Journal of theJapan Institute of Metals, vol. 59, no. 3 (1995), p. 284 to 289).

Further, Mg easily oxidizes. An oxide film of Mg causes deterioration ofthe wettability with Fe resulting in the adhesion deteriorating.

Even with Zn—Mg alloy plating or Zn—Mg—Al alloy plating, the activeamount of Zn or Al becomes smaller due to the added Mg. The formation ofan Zn—Fe alloy layer or Al—Fe alloy layer contributing to adhesionbetween the plating layer and Fe is therefore suppressed.

As a result, in Zn—Mg alloy plating, the higher the concentration of Mg,the more difficult it is to secure adhesion. At the time of working, theplating layer easily peels off etc. It was therefore only possible tofabricate an alloy plated steel material inferior in materialproperties.

The third reason why the Mg content has been kept low is that it hadbeen believed that with a plating composition containing Mg in a highconcentration, the corrosion resistance becomes poor.

Mg oxidizes the easiest among practical use metals, so even with alloyplating with an Mg concentration of a mass % of 50% or more, it had beenbelieved that the Mg would oxidize and the corrosion resistance wouldbecome poor and practicality would be lacking.

Due to these reasons, a steel material provided with a hot dip Znplating layer containing Mg in a high concentration has concerns interms of production and performance and has not existed up to now.

A method of producing plated steel sheet provided with a Zn—Mg alloyplating layer containing 35 mass % or more of Mg by electroplating isdisclosed in Japanese Patent Publication (A) No. 8-13186.

Up until now, the methods for producing plated steel material providedwith a Zn—Mg plating layer containing Mg in a high concentration haveall been inefficient methods such as electroplating methods using moltensalts or nonaqueous solvents. A method of production using the superiorefficiency hot dip plating method has not yet been proposed.

Further, the method of producing Zn—Mg plated steel sheet using thevapor deposition plating method utilizing the low melting point and highvapor pressure of Mg has been disclosed in “Nisshin Steel TechnicalReports, No. 78 (1998), 18-27”.

According to this method of production, it is believed possible toproduce plated steel sheet provided with a plating layer containing Mgin a high concentration, but vapor deposition in the order of Zn→Mg→Znis necessary. If compared with the hot dip plating method, it is aninefficient method of production.

Further, the concentration of Mg of the plating layer of a Zn—Mg platedsteel sheet produced by the method of production disclosed in “NisshinSteel Technical Reports, No. 78 (1998), 18-27” is 11 to 13 mass %. AMg—Zn alloy plating layer containing Mg in a high concentration is notbeing studied and its performance has not been disclosed at all.

The concentration of Mg of the plating layer of the hot dip plated steelmaterials disclosed up to now has been at most, by mass %, just 20%.Almost all research in this field has been limited to the range of Mg of20% or less.

Up to now, hot dip plating containing Mg in a high concentration hasactually never even come under study. Therefore, the properties of a hotdip plating layer containing Mg in a high concentration also have neverbeen clarified up to now.

DISCLOSURE OF THE INVENTION

The present invention has as its object the provision of a hot dip metalalloy plated steel material comprising a plated steel material providedwith a hot dip Mg—Zn alloy plating layer containing Mg in a highconcentration and achieving both adhesion and corrosion resistance.

The inventors studied the addition of Mg in a high concentration as ameans for obtaining a high corrosion resistance in hot dip Zn plating.

As a result, the inventors discovered that if setting the bathcomposition in a specific range of composition in an Mg-based-Zn platingbath containing Mg in a high concentration, it is possible to lower themelting point of the hot dip plating bath to less than the ignitionpoint of Mg and reduce both the viscosity of the plating bath and amountof production of dross and possible to produce a plated steel materialprovided with a hot dip Mg-based alloy plating layer. Note that“Mg-based-Zn” will sometimes be referred to below as “Mg—Zn”.

Further, the inventors investigated the physical properties andcross-sectional structure of this Mg—Zn alloy plating layer and as aresult discovered that in a low Mg alloy plating, formation of a Zn—Fealloy layer etc. contributing to plating adhesion was suppressed, but ifincluding Mg in a high concentration, if Zn is present in the platinglayer to a certain extent, the Fe diffuses from the matrix material tothe plating layer and enables adhesion to be secured.

Furthermore, they discovered that the adhesion of an Mg-based-Zn alloyplating layer with a steel sheet is further improved if preplating isapplied to the steel sheet with a metal film of Ni, Cu, Sn, etc.

Further, the inventors discovered that at part of the range ofcomposition of the present invention, it is possible to form anamorphous phase with a practical cooling rate and that if the amorphousphase becomes a volume percentage of 5% or more, defects forming thestarting points of peeling and cracking of the plating layer and thedetrimental effects of intermetallic compounds can be suppressed.

Further, the inventors discovered that the corrosion resistance of theMg-based alloy plating layer of the present invention is superior tothat of the conventional hot dip Zn plating layer, but by incorporatingan amorphous phase, the corrosion resistance is improved over a platinglayer of the same composition, but comprising only a crystal phasedepending on the conditions of use.

Even if the plating layer is not an amorphous, but crystal phase, inpart of the range of composition of the present invention, it ispossible to freeze the high temperature stable phase not existing theequilibrium state at room temperature as is until room temperature by apractical cooling rate.

Further, the inventors discovered that a plating layer containing thishigh temperature stable phase has an extremely superior corrosionresistance and sacrificial corrosion-proofing ability, so can beutilized as a high corrosion resistance and high sacrificialcorrosion-proofing ability plating layer never before existing in thepast.

The difficulty of forming a plating layer containing an amorphous phase,high temperature stable phase, or other nonequilibrium phase on thesteel sheet surface is due to the fact that after hot dip plating, it isnecessary to cool the plating layer by a large cooling rate.

The inventors studied targeting easily forming a hot dip Mg—Zn alloyplating layer containing this nonequilibrium phase on the steel sheetsurface and separating the hot dip plating process and cooling process.

As a result, they reached the series of heat processes of reheating andrapidly cooling hot dip Mg—Zn alloy plated steel sheet allowed tonaturally cool after plating (below, this reheating and rapid coolingsometimes being referred to as “reheating/rapid cooling”).

Usually, if plating, then reheating a plated steel material providedwith a hot dip plating layer containing Al or Zn, the Fe supplied fromthe plated steel material and the Al and/or Zn in the plating layer formintermetallic compound (alloy) layers (below, this being sometimesreferred to as “alloying”).

However, the inventors discovered that by reheating/rapid cooling byspecific temperature control in a specific range of composition in thehot dip Mg—Zn alloy plating layer of the present invention, it ispossible to suppress Fe and Al alloying or Fe and Zn alloying.

That is, in a specific range of composition, re-melt the plating layerwhile suppressing alloying is possible. If utilizing this, even with anordinary plating line not provided with the usual super rapid coolingfacilities, it is possible to first perform slow cooling to fabricate aplated steel material provided with an equilibrium phase hot dip Mg—Znalloy plating, then, off line or on line, reheat and rapidly cool thissteel material to produce plated steel sheet provided with anonequilibrium phase hot dip plating layer.

That is, by separating the rapid cooling process required for obtaininga nonequilibrium phase from the hot dip plating part, it becomespossible to easily form a nonequilibrium phase hot dip Mg—Zn alloyplating layer containing an amorphous phase or high temperature stablephase on the steel material.

The present invention was made based on the above discovery and has asits gist the following:

(1) An Mg-based alloy plated steel material characterized by beingprovided with a hot dip Mg-based alloy plating layer.

(2) An Mg-based alloy plated steel material characterized by beingprovided with a hot dip Mg-based alloy plating layer containing Zn: 15atm % to less than 45 atm %.

(3) An Mg-based alloy plated steel material characterized by beingprovided with a hot dip Mg-based alloy plating layer containing Zn: 15atm % to less than 45 atm % and further containing one or more elementsselected from a group of elements A: Si, Ti, Cr, Cu, Fe, Ni, Zr, Nb, Mo,and Ag in a total of 0.03 to 5 atm %.

(4) An Mg-based alloy plated steel material characterized by beingprovided with a hot dip Mg-based alloy plating layer containing Zn: 15atm % or more and Mg: over 35 atm % and further containing one or moreelements selected from the group of elements B: Al, Ca, Y, and La in atotal of 0.03 to 15 atm %.

(5) An Mg-based alloy plated steel material characterized by beingprovided with a hot dip Mg-based alloy plating layer containing Zn: 15atm % or more and Mg: over 35 atm % and further containing one or moreelements selected from the group of elements B: Al, Ca, Y, and La (B1)in a total of 0.03 to 15 atm % when Mg: over 55 atm % and (B2) in atotal of 2 to 15% when Mg: 55 atm % or less.

(6) An Mg-based alloy plated steel material as set forth in (4) or (5),characterized in that said hot dip Mg-based alloy plating layer containsMg: 85 atm % or less.

(7) An Mg-based alloy plated steel material as set forth in (4) or (5),characterized in that said hot dip Mg-based alloy plating layer containsMg: 55 to 85 atm %.

(8) An Mg-based alloy plated steel material as set forth in any one of(4) to (7) characterized in that said hot dip Mg-based alloy platinglayer further contains one or more elements selected from the group ofelements A: Si, Ti, Cr, Cu, Fe, Ni, Zr, Nb, Mo, and Ag in a total of0.03 to 5 atm %.

(9) An Mg-based alloy plated steel material as set forth in any one of(1) to (8) characterized in that said hot dip Mg-based alloy platinglayer contains Zn: 15 atm % to less than 45 atm % and contains anamorphous phase in a volume percentage of 5% or more.

(10) An Mg-based alloy plated steel material characterized by beingprovided with a hot dip Mg-based alloy plating layer containing Zn: 15atm % to less than 44.97 atm %, further containing one or more elementsselected from the composite group of elements of the group of elementsA: Si, Ti, Cr, Cu, Fe, Ni, Zr, Nb, Mo, and Ag and the group of elementsB′: Ca, Y, and La in a total of elements of the group of elements A of0.03 to 5 atm % and further a total of elements of the group of elementsB′ of 0.03 to 15 atm % (where, when said total is less than 0.03 to 5atm %, Mg: over 55 atm % and when 5 to 15 atm %, Zn: less than 40 atm%), and containing an amorphous phase in a volume percentage of 5% ormore.

(11) An Mg-based alloy plated steel material as set forth in any one of(1) to (8) characterized in that said hot dip Mg-based alloy platinglayer contains an intermetallic compound Zn₃Mg₇ in an X-ray intensityratio (ratio of diffraction peak intensity of Zn₃Mg₇ (excludingdiffraction peak of diffraction plane spacing of 0.233 nm) in the sum ofall diffraction peak intensities appearing at diffraction plane spacingof 0.1089 to 1.766 nm (excluding diffraction peak of diffraction planespacing of 0.233 nm)) of 10% or more.

(12) An Mg-based alloy plated steel material characterized by beingprovided with a hot dip Mg-based alloy plating layer containing Zn: 20atm % or more and Mg: 50 atm % to 75 atm %, further containing one ormore elements selected from the group of elements B: Al, Ca, Y, and Lain a total of 0.03 to 12 atm % (where when said total is 1 to 12 atm %,containing Al: 1 atm % or more), and containing an intermetalliccompound Zn₃Mg₇ in a required amount.

(13) An Mg-based alloy plated steel material as set forth in any one of(1) to (8) characterized in that said hot dip Mg-based alloy platinglayer contains a nonequilibrium phase obtained by holding said platinglayer at a temperature of a melting point of the Mg-based alloy platingto (melting point of Mg-based alloy plating+100° C.) for 1 minute orless, then rapidly cooling it.

(14) An Mg-based alloy plated steel material as set forth in (13),characterized in that said nonequilibrium phase is one or both of anamorphous phase and intermetallic compound Zn₃Mg₇.

(15) An Mg-based alloy plated steel material as set forth in (13) or(14) characterized in that said rapid cooling is water cooling or mistwater cooling.

(16) An Mg-based alloy plated steel material as set forth in any one of(1) to (15) characterized in that the interface between said hot dipMg-based alloy plating layer and steel material is provided with apreplating layer comprised of one or more elements selected from Ni, Cu,Sn, Cr, Co, and Ag.

(17) An Mg-based alloy plated steel material as set forth in any one of(1) to (16) characterized in that said hot dip Mg-based alloy platinglayer contains a balance of Mg and unavoidable impurities.

The present invention (Mg-based alloy plated steel material) enablesproduction by the usual hot dip plating process, so is superior inuniversality and economy.

Further, the hot dip Mg—Zn alloy plating layer of the present inventionenables a corrosion resistance superior to a conventional hot dip Znplating layer while keeping down the concentration of Zn, so contributesto the saving of Zn resources.

Further, the hot dip Mg-based alloy plating layer of the presentinvention is excellent not only in corrosion resistance, but also inworkability, so the present invention can be widely used as a structuralmember or functional member in automobiles, building materials, andhousehold electric appliances.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view showing the region of composition where the meltingpoint becomes 580° C. or less due to the addition of Al, Ca, Y, and/orLa.

FIG. 2 is a view showing the region of composition where the meltingpoint becomes 520° C. or less due to the addition of Al, Ca, Y. and/orLa.

FIG. 3 is a view showing the region of composition where an amorphousphase is obtained.

FIG. 4 is a view showing a binary alloy phase diagram of Mg—Zn.

FIG. 5 is a view showing the region of composition where Zn₃Mg₇ isobtained.

FIG. 6 is a view showing the cross-sectional structure of an Mg-25 atm %Zn-5 atm % Ca plating layer (crystal phase).

FIG. 7 is a view showing the cross-sectional structure of an Mg-25 atm %Zn-5 atm % Ca plating layer (amorphous phase).

FIG. 8 is a view showing an X-ray diffraction pattern of an Mg-25 atm %Zn-5 atm % Ca plating layer (amorphous phase).

FIG. 9 is a view showing an FE-TEM image (bright field image) near theinterface of an Mg-25 atm % Zn-5 atm % Ca plating layer (amorphousphase).

FIG. 10 is a view showing the results of elemental analysis by EDX atthe cross point in the FE-TEM image shown in FIG. 9.

FIG. 11 is a view showing an electron beam diffraction pattern at thecross point in the FE-TEM image shown in FIG. 9.

FIG. 12 is a view showing an X-ray diffraction pattern of an Mg-25 atm %Zn-5 atm % Ca-4 atm % Al plating layer of No. 16 in Table 9 (amorphousphase, Zn₃Mg₇).

FIG. 13 is a view showing an X-ray diffraction pattern of an Mg-27 atm %Zn-1 atm % Ca-6 atm % Al plating layer (Zn₃Mg₇) of No. 3 in Table 9.

FIG. 14 is a view showing the X-ray diffraction pattern of an Mg-27 atm% Zn-1 atm % Ca-6 atm % Al plating layer of No. 3 in Table 9 (in theFIG. 10), the X-ray diffraction pattern of an Mg-27 atm % Zn-1 atm %Ca-8 atm % Al plating layer of No. 6 (in the FIG. 11), the X-raydiffraction pattern of an Mg-27 atm % Zn-1 atm % Ca-10 atm % Al platinglayer of No. 7 (in the FIG. 12), and the X-ray diffraction pattern of anMg-27 atm % Zn-1 atm % Ca-13 atm % Al plating layer of No. 8 (in theFIG. 13).

FIG. 15 is a view showing the mode of a cycle corrosion test.

FIG. 16 is a view showing the appearances of corrosion in the results ofcycle corrosion tests according to the invention test materials andcomparative test materials.

FIG. 17 is a view showing the corrosion morphology in the cross-sectionof the steel sheet of the Comparative Test Material 1.

FIG. 18 is a view showing the corrosion morphology in the cross-sectionof the steel sheet of the Comparative Test Material 2.

FIG. 19 is a view showing the corrosion morphology at the cross-sectionof the corrosion products of the Invention Test Material 1 (up to 21cycles).

FIG. 20 is a view showing the corrosion morphology at the cross-sectionof the corrosion products of the Invention Test Material 1 (after 21cycles to 56 cycles).

FIG. 21 is a view showing the corrosion morphology at the cross-sectionof the corrosion products of the Invention Test Material 2 (up to 21cycles).

FIG. 22 is a view showing the corrosion morphology at the cross-sectionof the corrosion products of the Invention Test Material 2 (after 21cycles to 56 cycles).

FIG. 23 is a view showing the results of examination of thecross-section of the corrosion products formed at 42 cycles of theInvention Test Material 1 by EPMA.

FIG. 24 is a view showing the results of examination of thecross-section of the corrosion products formed at 42 cycles of theInvention Test Material 2 by EPMA.

FIG. 25 is a view showing a phase diagram of an Al—Mg alloy.

FIG. 26 is a view showing a phase diagram of a Cu—Mg alloy.

FIG. 27 is a view showing a phase diagram of an Ni—Mg alloy.

BEST MODE FOR CARRYING OUT THE INVENTION

Below, the present invention will be explained in detail.

Inherently, Mg is a metal extremely difficult to deposit on a steelmaterial by the hot dip plating method. This is due to the fact that (i)Mg does not react much at all with Fe and, further, (ii) Mg does notdissolve much in Fe (even if dissolving, about 10 ppm), that is, thepoor compatibility of the elements.

For this reason, conversely, it is possible to utilize the poorcompatibility to use the steel material as is as a “crucible” materialfor melting Mg. That is, if using a steel “crucible” for melting the Mg,the “crucible” will not be damaged and the molten Mg can be maintained.

Due to the above reasons and the property of the activity of Mg ofeasily igniting at the melting point, it has not been possible to forman Mg plating layer or a plating layer of an Mg-based alloy containingMg in a high concentration (for example, an Mg-based-Zn alloy) on asteel material by the hot dip plating method.

However, Mg is a metal with a low corrosion potential and an extremelysuperior sacrificial corrosion-proofing effect for a steel material. Theinventors took note of this superior point and intensively researchedthe technique of forming a plating layer of an Mg-based alloy (forexample, Mg-based-Zn alloy) containing Mg in a high concentration on thesurface of a steel material by the hot dip plating method. As a result,they discovered that

(x) If using a plating bath of an Mg-based-Zn alloy comprised of Mg intowhich a required amount of Zn is added so as to plate steel sheet, it ispossible to form an Mg-based-Zn alloy plating layer superior in adhesionwith steel sheet on the steel sheet surface.

Note that below, the “alloy plating layer” and “plating layer”, unlessotherwise particularly explained, mean an “alloy plating layer comprisedof a crystal phase” and a “plating layer comprised of a crystal phase”.

In the method of formation of an Mg-based alloy plating layer of thepresent invention, the technique of adding Zn to Mg is employed based onthe above discovery (x). That is, in the present invention, thetechnique of “adding Zn to Mg” forms the basis of the present invention.

If trying to form the Mg-based alloy plating layer of the presentinvention by the conventional technique of adding high concentration Mgto Zn, along with the increase in the amount of addition of Mg, asexplained above, the amount of production of MgZn₂ increases, themelting point of the plating bath rises, and the viscosity of theplating rises. Dissolution of Mg into Zn is no longer possible at acertain concentration. The undissolved remaining Mg ends up igniting inthe atmosphere.

On the other hand, when adding Zn to Mg like with the technique ofaddition of the present invention, the above-mentioned phenomenon doesnot arise. Adding Zn to Mg has not been considered up to now, but theinventors engaged in intensive research and as a result discovered atechnique of adding Zn to Mg.

If adding Zn to Mg, since (Mg: 70 atm %-Zn: 30 atm %) is a eutecticcomposition, if the amount of addition of Zn increases, the viscosity ofthe plating bath falls.

As Mg alloys exhibiting phase diagrams similar to a phase diagram ofZn—Mg, there are an Al—Mg alloy, Cu—Mg alloy, and Ni—Mg alloy. Forreference, FIG. 25 shows a phase diagram of Al—Mg alloy, FIG. 26 shows aphase diagram of a Cu—Mg alloy, and FIG. 27 shows a phase diagram ofNi—Mg alloy.

As will be understood from these figures, if adding Al, Cu, or Ni in anamount of 10 to 30 atm %, a eutectic composition with Mg is formed. Theeutectic composition differs in atomic ratio with the eutecticcomposition of the Mg—Zn alloy, but Al, Cu, and Ni are elements providedwith similar functions to Zn, the inventors believed.

The reason why it was not possible to add a high concentration of Mg toZn up to now was that at the time of addition of Mg, the intermetalliccompound MgZn₂ was produced, but in the present invention, the techniqueof adding Zn to Mg was employed to avoid the production of MgZn₂ andtherefore formation of an Mg-based-Zn alloy plating layer containing Mgin a high concentration on the steel material surface became possible.

To more simply add Zn to Mg, first, a small amount of high Mg—Zn ingotis prepared in an argon atmosphere. This ingot is melted in theatmosphere and Mg and Zn are alternately added to increase the meltedamounts so as not to greatly deviate from the eutectic composition (Mg:70 atm %, Zn: 30 atm %).

The eutectic composition Mg—Zn alloy melts near 350° C., so it ispossible to avoid ignition of the Mg (ignition point 560° C.). Themelting of the Mg in the atmosphere is accompanied with the danger ofstarting fires and explosions, so it is preferable to melt it as much aspossible in an argon atmosphere or other inert atmosphere. However, theamount of the Mg—Zn alloy targeted is large, so when it is not possibleto prepare the entire targeted amount of Mg—Zn alloy in an argonatmosphere, it is preferable to employ the technique of preparing onlythe seed alloy in an argon atmosphere in the above way, then alternatelyadd Mg and Zn in the atmosphere.

Note that to suppress the ignition of Mg and the production of blackoxides, it is sufficient to add Ca to the Mg at the same time as addingthe Zn. The reason why Mg stabilizes due to the addition of Ca is, asone reason, that Ca oxidizes more easily than Mg.

The inventors used the Mg-based alloy plating bath prepared by themethod of addition of the present invention so as to form an Mg-basedalloy plating layer on a steel sheet and investigated the corrosionmorphology at said plated steel sheet.

Furthermore, they compared the results of their investigations and thecorrosion morphology in conventional hot dip Zn alloy plated steelsheet.

They subjected the present invention and conventional plated steelsheets to cycle corrosion tests to investigate them.

FIG. 15 shows the modes of the cycle corrosion test.

The cyclic corrosion test used here is a corrosion test developed so asto match relatively well the actual state of corrosion in generalexposure test. The development was carried out by lowering the saltconcentration in the salt spray process of an accelerated corrosion testthat had been established as a corrosion test matching well with theactual state of corrosion state of steel sheet for automobiles.

The inventors ran the cycle tests and as a result learned that thecorrosion morphology in an Mg-based alloy plated steel material of thepresent invention substantively differs in the corrosion morphology inconventional hot dip Zn alloy plated steel material. Specifically, theylearned the following:

(y) In a plating layer with a sufficiently high Mg concentration, mostof the corrosion products are Mg(OH)₂, basic magnesium carbonate, andother “corrosion products mainly comprised of Mg”.

(z) The “corrosion products mainly comprised of Mg” exhibit a farstronger effect of protection of the iron metal compared with corrosionproducts mainly comprised of Zn and remarkably suppress the formation ofred rust even after all of the plating metal changes to corrosionproducts.

Here, part of the results of the cycle corrosion tests until obtainingthe discovery (y) and the discovery (z) will be explained.

The inventors used the following four types of test materials for cyclecorrosion tests:

(1) Steel sheet provided with a 68 atm % Mg-27 atm % Zn-5 atm % Ca alloyplating layer (amorphous, layer thickness: 10 μm) (Invention TestMaterial 1)

(2) Steel sheet provided with a 68 atm % Mg-27 atm % Zn-5 atm % Ca alloyplating layer (crystalline, layer thickness: 10 μm) (Invention TestMaterial 2)

(3) Steel sheet provided with a hot dip Zn plating layer (layerthickness: 14 μm) (Comparative Test Material 1 (commercially availablematerial))

(4) Steel sheet provided with a hot dip Zn—Al—Mg alloy plating layer(layer thickness: 12 μm) (Comparative Test Material 2 (commerciallyavailable material))

FIG. 16 shows the appearances of corrosion of the results of cyclecorrosion tests according to the Invention Test Materials 1 and 2 andComparative Test Materials 1 and 2.

In Comparative Test Material 1, at 28 cycles, red rust forms on thesteel sheet surface and corrosion of the iron metal also occurs. In theother test materials, the surface is covered by the corrosion productsand corrosion of the iron metal does not occur.

At 56 cycles, in Comparative Test Material 2, red rust forms on thesteel sheet surface and corrosion of the iron metal also occurs. On theother hand, in the Invention Test Materials 1 and 2, red rust does notform on the steel sheet surface and the iron metal is protected.

From these, it will be understood that the hot dip Mg-based alloyplating layer of the present invention is remarkably superior to aconventional Zn plating layer and Zn alloy plating layer in corrosionresistance and sacrificial corrosion-proofing ability.

Next, the inventors examined the cross-section of the plated steel sheetby an optical microscope to examine the corrosion morphology. FIG. 17 toFIG. 20 show the results.

FIG. 17 shows the corrosion morphology at the cross-section of the steelsheet of the Comparative Test Material 1 provided with a hot dip Znplating layer (layer thickness: 14 μm). At 14 cycles, red rust isformed. Further, from the cross-section at 21 cycles, it is learned thatafter the formation of red rust, the iron metal rapidly corrodes.

FIG. 18 shows the corrosion morphology at the cross-section of the steelsheet of the Comparative Test Material 2 provided with a hot dipZn—Al—Mg alloy plating layer (layer thickness: 12 μm). At 56 cycles, redrust is formed. The corrosion rate of the plating layer is slow, butthere is little protective action of the iron metal by the corrosionproducts. Even if corrosion products form, the iron metal corrodes.

FIG. 19 shows the corrosion morphology up to 21 cycles at thecross-section of the steel sheet of the Invention Test Material 1provided with a 68 atm % Mg-27 atm % Zn-5 atm % Ca alloy plating layer(amorphous, layer thickness: 10 μm), while FIG. 20 shows the corrosionmorphology from 21 cycles to 56 cycles.

As shown in FIG. 19, at 14 cycles, a small amount of the corrosionproducts A was formed. After this, at the cross-section at 21 cycles,the corrosion products B formed little by little from the corrosionproducts A.

During this time, the advance of corrosion rate is fast at the amorphousphase. As shown in FIG. 20, at 28 cycles, where the corrosion products Breaches 20 μm, the plating layer ends up becoming almost completely acorrosion layer.

This does not mean that the corrosion resistance of the amorphousplating layer rapidly changes. Part of the corrosion of the platinglayer reaches the steel sheet, whereby the sacrificialcorrosion-proofing ability acts strongly and the corrosion rate of theplating layer is accelerated. By increasing the plating layer thickness,it is also possible to make the corrosion resistance at the start of thecycle corrosion test excellent.

However, after this, the corrosion stops. Even by 42 cycles and the next56 cycles, the iron metal does not corrode.

When the plating layer is an amorphous layer, time is required for theformation of the corrosion products B with a high protective ability,but in the end, the corrosion products become a two-layer structure ofthe corrosion products A and the corrosion products B and suppresses thecorrosion of the iron metal.

FIG. 23 shows the result when observing the cross-section of thecorrosion products formed by 42 cycles of the Invention Test Material 1by EPMA. At the time of 42 cycles, the plating layer of the InventionTest Material 1 becomes the two-layer state of the corrosion products Aand the corrosion products B.

At the lower layer corrosion products A, the Cl concentration and Oconcentration are high. On the other hand, the Zn concentration, Mgconcentration, and Ca concentration are average concentrations. On theother hand, at the upper layer corrosion products B, the Cconcentration, O concentration, and Mg concentration are extremely high.

From these results, the corrosion products A are comprised of an oxideor chloride of Zn, Mg, and Ca. On the other hand, the corrosion productsB can be deduced to be comprised of Mg carbonate compounds.

Therefore, the corrosion-proofing effect of the Mg-based alloy platingcan be guessed to probably be due to the Mg carbonate compounds.

Note that at the point of 42 cycles, in the plating layer, corrosionfront reaches the interface between the plating layer and iron metal,but it was learned that no dissolution of Fe occurred at all.

FIG. 21 shows the corrosion morphology up to 21 cycles in thecross-section of the steel sheet of the Invention Test Material 2provided with a 68 atm % Mg-27 atm % Zn-5 atm % Ca alloy plating layer(crystalline, layer thickness: 10 μm), while FIG. 22 shows the corrosionmorphology from after 21 cycles to 56 cycles.

When the plating layer is crystalline, at the start, the corrosionproducts A are formed and cover the entire plating layer surface (see 7cycles). At this time, the loss of thickness by corrosion is about 5 μm.This corrosion rate is the same as the case of a hot dip Zn platinglayer (Comparative Test Material 1).

However, the corrosion products B are immediately formed from thecorrosion products A (see 14 cycles) whereby the corrosion of theplating layer and the iron metal is suppressed.

The plating layer corrodes a little at a time, but in the middle, theplating loss become equal to that of the amorphous phase layer where ittakes time until the corrosion products B are form. In some cases, thecorrosion loss of the crystalline plating layer may even become smaller(see 28 cycles of FIG. 22).

As shown in FIG. 22, at 42 cycles and 56 cycles, the plating layerchanges almost completely to the corrosion products A, but in the sameway as the amorphous plating layer, the corrosion stops and no corrosionof the iron metal occurs.

FIG. 24 shows the results of observation of the cross-section of thecorrosion products formed by 42 cycles of the Invention Test Material 2by EPMA. The plating layer of the Invention Test Material 2, in the sameway as the plating layer of the Invention Test Material 1, is a twolayer state of the corrosion products A and the corrosion products B.

From the figures, it will be understood that Cl, O, Zn, Mg, and Ca arestrongly detected from the corrosion products A, and C, O, and Mg aredetected from the corrosion products B.

From this, the corrosion products formed are believed to be ones similarto the corrosion products formed in Invention Test Material 1.

In the end, when the plating layer is crystalline, the highly protectivecorrosion products B are immediately formed at a relatively early stage,so corrosion rate is fast at the early stage, but slows in the middlestage of corrosion.

Finally, the corrosion products become a two-layer structure of thecorrosion products A and the corrosion products B and suppress corrosionof the iron metal.

As explained above, the corrosion morphology in the Mg-based alloyplated steel material of the present invention actually differs from thecorrosion morphology in the conventional hot dip Zn alloy plated steelmaterial.

Next, that the reasons for limiting the composition of ingredients ofthe hot dip Mg-based alloy plating layer of the present invention willbe explained.

To secure adhesion between the plating layer and steel material in a hotdip Mg—Zn alloy plating layer, it is necessary to make Fe diffuse in theplating layer. For this reason, it is necessary to include Zn in the hotdip plating bath. Zn has to be 15 atm % or more.

Note that below, unless the % is particularly explained, the % showingthe compositions means atm %.

If Zn is less than 15%, the activity of Zn in the plating bath becomesinsufficient, sufficient Fe diffusion does not occur, and sufficientadhesion cannot be obtained between the plating layer and the steelmaterial. Due to the diffusion, sometimes the Fe is contained in theentire plating layer up to 3% or so.

However, the diffused concentration of Fe becomes higher at theinterface between the plating layer and the steel sheet. When thethickness of the plating layer is small, the diffused concentration ofFe becomes higher.

Here, the 3% in the case of increasing the Fe concentration is theconcentration when the thickness of the plating layer is 10 μm or so. Toimprove the adhesion of the plating layer, diffusion of Fe, even slight,is required, but the amount need only be 0.1% in a plating layer with athickness of about 10 μm.

By having the Mg contain Zn in 15% to less than 45%, the melting pointof Mg remarkably falls and becomes 520° C. or less. This is due to thefact that (Mg: 70%-Zn: 30%) is a binary (Mg—MgZn₂) eutectic composition.

The melting point of a eutectic composition is lower than the ignitionpoint of Mg, that is, about 520° C., so even if performing Mg-basedalloy plating in the atmosphere, the Mg will not ignite. For thisreason, a binary (Mg—MgZn₂) eutectic composition is the optimumcomposition as a plating condition.

If Zn becomes 45% or more, the result ends up becoming far from a binaryeutectic composition, the amount of production of MgZn₂ increases, themelting point of the plating bath rises, and the viscosity alsoincreases. Further, if Zn is 45% or more, the melting point of theplating bath is liable to exceed the ignition point, so Zn has to bemade less than 45%.

The corrosion resistance of the hot dip Mg-based alloy plating layer ofthe present invention is superior to the corrosion resistance of a hotdip Zn plated layer of a hot dip Zn plated steel sheet. The corrosionpotential of the hot dip Mg-based alloy plating layer of the presentinvention is −1.0 to −1.5V (in 0.5% NaCl aqueous solution, vs. Ag/AgCl).The sacrificial corrosion-proofing ability with respect to the steelmaterial is also remarkably superior.

That is, the hot dip Mg-based alloy plating layer of the presentinvention is far superior to the conventional hot dip Zn plating layerin corrosion resistance and sacrificial corrosion-proofing ability.

For the purpose of further raising the corrosion resistance of a hot dipMg-based alloy plating, one or more elements selected from Fe, Cr, Cu,Ag, Ni, Ti, Zr, Mo, Si, and/or Nb (group of elements A) are added to theplating bath.

If these elements are added in a total of 0.03% or more, the corrosioncurrent density near the corrosion potential of the polarization curveobtained by electrochemical measurement begins to become smaller.

If the total amount of addition of the above elements is over 5%, themelting point of the plating bath rises and plating becomes difficult,so the total amount of the elements of the group of elements A added tothe plating bath is preferably 5% or less.

One or more elements selected from Al, Ca, Y, and/or La (group ofelements B) also are suitably added to the plating bath to improve thecorrosion resistance. If adding a total of up to 10%, the melting pointand viscosity of the plating bath fall.

With addition of a total of 0.03% or more, the corrosion current densitynear the corrosion potential of the polarization curve obtained byelectrochemical measurement begins to become smaller and the corrosionresistance of the plating layer is improved, but if the total amount ofaddition exceeds 15%, the melting point of the plating bath becomeshigher, so the total amount of addition of the elements of the group ofelements B added to the plating bath is preferably 15% or less.

Further, due to the addition of Al, Ca, Y, and/or La, the melting pointand viscosity of the Mg—Zn alloy fall, so even if Zn is 45% or more, themelting point of the plating bath becomes less than the ignition pointof Mg of 520° C. and there is a range of composition where Mg-basedalloy plating in the atmosphere becomes possible.

Note that due to the addition of Al, Ca, Y, and/or La, the ignitionpoint of the Mg—Zn alloy rises to about 580° C.

FIG. 1 shows the region of composition where the melting point becomes580° C. or less due to the addition of Al, Ca, Y, and/or La. In thefigure, 1 is the binary (Mg—MgZn₂) eutectic line and 2 is the ternaryeutectic line.

If Zn is 15% or more, Mg is over 35%, and the total amount of additionof Al, Ca, Y, and/or La is 0.03 to 15%, the viscosity of the platingbath is low and the melting point becomes 580° C. or less.

By further limiting the region of composition shown in FIG. 1, themelting point can be made 520° C. or less. FIG. 2 shows the region ofcomposition where the melting point becomes 520° C. due to the additionof Al, Ca, Y, and/or La.

If Zn is 15% to less than 45%, Mg is over 35%, and the total amount ofaddition of Al, Ca, Y, and/or La is 0.03 to 15%, the viscosity of theplating bath is low and the melting point becomes 520° C. or less.

Even if Zn is 45% or more, if Mg is over 35% and the total amount ofaddition of Al, Ca, Y, and/or La is 2 to 15%, the viscosity of theplating bath is low and the melting point becomes 520° C. or less.

The total amount of addition of the elements of the group of elements Bis made 0.03 to 15% since it is believed that near the elementconcentration of 7.5%, there is a ternary eutectic line formed by theelements of the group of elements B, Mg, and MgZn₂ (in FIG. 2, see “2”),and the liquid state of the Mg—Zn alloy stabilizes near this ternaryeutectic composition.

For this reason, even if Zn is 45% or more and the plating is far from abinary eutectic composition, it is possible to approach a ternaryeutectic line by the addition of elements of the group of elements B andthe liquid state of the Mg—Zn alloy stabilizes.

However, if adding the elements of the group of elements B in a total ofover 15%, the plating ends up greatly deviating from the ternaryeutectic line, the melting point of the Mg—Zn alloy rises, and Mg-basedalloy plating becomes difficult, so the upper limit of the total amountof addition of the elements of the group of elements B is preferably15%.

Further, if Mg becomes 35% or less, there is soon no longer a eutecticline. Even if adjusting the amount of addition of the group of elementsB, the amounts of production of MgZn₂, CaZn₅, etc. increase, the meltingpoint of the plating bath becomes 520° C. or more, and Mg-based alloyplating becomes difficult. Therefore, the lower limit of Mg becomes over35%.

In the case of an Mg—Zn alloy plating, if raising the cooling rate inthe range of composition of Zn of 15% to less than 45%, it is possibleto obtain an amorphous phase.

If the plating layer contains an amorphous phase in an amount, by volumepercentage of the plating layer, of 5% or more, the corrosion resistanceof the plating layer is superior to the corrosion resistance of aplating layer of the same composition, but comprising only a crystalphase.

If the amorphous phase is present in the plating layer, the corrosionpotential becomes more noble compared with the corrosion potential of aplating layer of the same composition, but comprising only a crystalphase.

If the plating layer contains an amorphous phase in 5 vol % or more, thecorrosion potential rises by 0.01V or more compared with the corrosionpotential of a plating layer of the same composition, but comprisingonly a crystal phase. Further, the corrosion current density at thecorrosion potential also becomes smaller.

The corrosion resistance in an actual environment can be evaluated by acycle corrosion test. A plating layer containing an amorphous phase inan amount of 5 vol % or more as a result of the evaluation has less of acorrosion loss at the start of a cycle corrosion test than a platinglayer of the same composition, but comprising only a crystal phase.

If the plating layer contains an amorphous phase in an amount, by volumepercentage, of less than 5%, the plating layer exhibits a corrosionresistance equal to that of a plating layer of the same composition, butcomprising only a crystal phase (plating layer cooled by nitrogen gasafter plating).

The rise of the corrosion potential is less than 0.01V, the corrosioncurrent density also becomes substantially equal, and no clear change incharacteristics can be seen. The evaluation of the corrosion resistanceby a cycle corrosion test was similar.

The reasons why the corrosion resistance is improved if the platinglayer contains an amorphous phase are not clear, but it is believed that(a) the amorphous phase is a homogeneous structure with neither crystalgrain boundaries where the elements segregate nor intermetalliccompounds, (b) elements improving the corrosion resistance can bedissolved in the matrix phase up to the solution limit, and (c) anamorphous phase is a nonequilibrium phase, so the surface is activatedand a dense oxide film is rapidly formed.

Furthermore, when forming a plating layer containing an amorphous phase,if adding Ca, Y, and/or La (group of elements B′), the amorphous phaseforming ability derived from the composition of the plating layer isimproved.

If adding elements of the group of elements B′ raising the amorphousphase forming ability to the plating bath, it is possible to form a hotdip Mg-based alloy plating layer containing an amorphous phase on asteel sheet easily.

The group of elements B′ feature giant atoms compared with Zn and Mg. Toraise the amorphous phase forming ability, it is sufficient that atomswhich would inhibit movement of atoms at the time of solidification areincluded in the alloy so that the liquid state becomes as stable aspossible.

As such atoms, in addition to Ca, Y, and La, Ce, Yb, and otherrelatively large atom size lanthanide elements may be mentioned. Theseelements are considered to exhibit actions and effects similar to thegroup of elements B.

Addition of Al is effective for improvement of the corrosion resistance,but has no action in improving the amorphous phase forming ability.

This is believed to be because the enthalpy of formation of liquid of Alwith Zn is positive and Al is an element with properties different fromthe Ca, Y, and/or La with negative liquid enthalpy with Zn.

The compositions giving an amorphous phase in a hot dip Mg-based alloyplating layer are limited.

FIG. 3 shows a region of composition giving an amorphous phase. Acomposition giving an amorphous phase is limited to specificcompositions due to the difference between the melting point and glasstransition temperature of the Mg-based alloy.

Even if the composition of ingredients changes, the glass transitiontemperature will not change that much, so an amorphous phase is usuallyeasily formed the lower the melting point. Therefore, the amorphousphase forming ability is closely related to the eutectic composition.

A eutectic composition Mg-based alloy has a low melting point, so is acomposition most easily maintaining its liquid state down to the glasstransition temperature.

In a composition comprised of elements selected from Mg, Zn, and thegroup of elements B′, the eutectic line cross point 3 (see “3” in FIG.3) where the binary (Mg—MgZn₂) eutectic line and the ternary eutecticline cross is the lowest in melting point. In the region of compositionnear this cross point, the amorphous phase forming ability becomesextremely high.

If Mg becomes 55% or less in a hot dip Mg-based alloy plating layercontaining elements of the group of elements B′ in a total of less than5%, the plating becomes far from a eutectic composition, the meltingpoint rises, and the amorphous phase forming ability becomes smaller.

As a result, it becomes difficult to form an amorphous phase in aplating layer by a plating process using water cooling, so when formingan amorphous phase, Mg is made over 55%.

In the same way, if Zn becomes 40% or more in an alloy plating layercontaining elements of the group of elements B′ in a total of 5% ormore, the plating becomes far from a eutectic composition, the meltingpoint rises, and the amorphous phase forming ability becomes smaller.

As a result, it becomes difficult to form an amorphous phase in aplating layer by a plating process using water cooling, so when formingan amorphous phase, Zn is made less than 40%.

In a range of composition of Zn: less than 40% and Mg: over 55%, themelting point becomes a remarkably low 450° C. or less, so this range ofcomposition is the range of composition convenient for obtaining anamorphous phase.

Further, by including an amorphous phase in a hot dip Mg-based alloyplating layer containing elements of the group of elements A, it ispossible to further improve the corrosion resistance.

Utilizing the effect of improvement of the corrosion resistance due tothe addition of corrosion resistance improving elements and formation ofan amorphous phase, it is possible to produce steel sheet provided witha hot dip Mg-based alloy plating layer remarkably superior in corrosionresistance.

The hot dip Mg-based alloy plating layer of the present invention andthe hot dip Mg-based alloy plating layer containing an amorphous phaseare plating layers superior in both workability and adhesion. An Mg—Znalloy is an alloy with extremely slow crystallization and grain growth.

For this reason, in the plating layer, by just slightly raising thecooling rate, the crystal grains easily become finer, so it is possibleto reduce the detrimental effect of intermetallic compounds poor inplastic deformability on the workability and adhesion.

If it is possible to obtain an amorphous phase having an atomicstructure of a liquid state, the intermetallic compounds are eliminated,so the workability and adhesion can be further improved.

In hot dip Mg—Zn alloy plating, in addition to the technique of formingan amorphous phase in the plating layer, it is possible to strikinglyimprove the corrosion resistance by the presence of an intermetalliccompound phase of Zn₃Mg₇.

Zn₃Mg₇ (Zn₃Mg₇ is expressed as Mg₅₁Zn₂₀ in some papers, but in thepresent description the two intermetallic compounds are treated as thesame substances and are all expressed as Zn₃Mg₇) is a high temperaturestable phase as shown in FIG. 4.

For this reason, if applying slow cooling as in an ordinary hot dipplating process, the Mg and Zn in the molten state separate into an Mgphase and MgZn or Mg₄Zn₇. It is not possible to leave Zn₃Mg₇ at anordinary temperature.

However, in the same way as forming an amorphous phase, it is possibleto leave Zn₃Mg₇ by rapid cooling (for example, water cooling or mistcooling) right after hot dip plating.

Zn₃Mg₇ can be formed even in a composition with a small amorphous phaseforming ability, that is, Mg—Zn alloy plating or Mg—Zn—Al alloy plating.

In a composition with a high Ca concentration in an Mg—Zn—Al—Ca alloyplating, if water cooling after hot dip plating, sometimes an amorphousphase and Zn₃Mg₇ are mixed in a plating layer.

FIG. 5 shows the range of composition by which Zn₃Mg₇ is obtained by hotdip plating, then water cooling. The range of composition shown in FIG.5 is the range of composition where Zn₃Mg₇ can be easily detected as theXRD peak by X-ray diffraction of the plated steel sheet surface.

This range of composition is the range of composition where the X-rayintensity ratio (ratio of diffraction peak intensity of Zn₃Mg₇(excluding diffraction peak of diffraction plane spacing of 0.233 nm) inthe sum of all diffraction peak intensities appearing at diffractionplane spacings of 0.1089 to 1.766 nm, that is, diffraction angles 2θ of5 to 90° in case of diffraction measurement by Cu—Kα rays using an X-raytube with Cu target for the X-ray source (however, excluding diffractionpeak of diffraction plane spacing of 0.233 nm)) is 10% or more.

The diffraction peak of a diffraction plane spacing of 0.233 nm ispreferably excluded since the strongest line of Mg and the diffractionpeak are close. Note that the diffraction peak of Zn₃Mg₇ is found byreferring to the diffraction data charts (JCPDS card no.: 08-0269).

To form Zn₃Mg₇, it is necessary that Zn be 20% or more, Mg be 50% to75%, and the one or more elements selected from the group of elements B:Al, Ca, Y, and La be a total of 0.03 to 12%. In the range of compositionwhere the Ca concentration or Y and La concentration is high and theamorphous phase forming ability is high, sometimes an amorphous phase isformed and Zn₃Mg₇ cannot be obtained.

In particular, when using water cooling (water immersion) as a rapidcooling method, only a certain cooling rate can be obtained, soobtaining a Zn₃Mg₇ phase is difficult. Even with a composition where anamorphous phase is generally obtained, by changing the rapid coolingmethod from water cooling (for example, to mist cooling etc.) so as toreduce the cooling rate given to the plating layer, it is possible topartially obtain a Zn₃Mg₇ phase. Below, unless otherwise indicated, thecase of use of water cooling as the rapid cooling method is assumed.

Therefore, when Ca, Y, and/or La exceed a total of 1%, it is necessaryto add Al: 1% or more and prevent the amorphous phase forming abilityfrom being raised too much.

Al is an element promoting the formation of Zn₃Mg₇ more than theamorphous phase, so if the Al concentration is higher than the Caconcentration, Zn₃Mg₇ is more easily formed than the amorphous phase.

When Ca, Y, and/or La are a total of not more than 1%, formation of asmall amount of amorphous phase and formation of Zn₃Mg₇ occursimultaneously.

If Zn₃Mg₇ is contained in the plating layer, the corrosion potential ofthe plating layer becomes about −1.2V (vs. Ag/AgCl) in a 0.5% NaClaqueous solution.

This value is a high value compared with the corrosion potential of −1.5to −1.4V of a plating layer of the same composition but not containingZn₃Mg₇ (plating layer air cooled after plating). The greater the amountof Zn₃Mg₇ in the plating layer, the closer the corrosion potential to−1.2V. The corrosion current density near the corrosion potential of thepolarization curve starts to become smaller.

Even in a plating layer where Zn₃Mg₇ is detected by X-ray diffraction,if Al or Ca is added in the plating layer, the corrosion current densitybecomes small. With Al of 0 to 6% or so, if the concentration increases,the corrosion current density becomes smaller. If adding Ca: 0.3 to 5%,the corrosion current density becomes smaller.

When desiring to make Zn₃Mg₇ precipitate preferentially over theamorphous phase, Al is added in a greater amount than Ca.

Zn₃Mg₇ remarkably raises the corrosion resistance of the plating layer,but if present in a large amount in the plating layer, the workabilityof the plating layer degrades and cracking easily occurs.

On the other hand, an amorphous phase does not have as much of an effectof improvement of the corrosion resistance as Zn₃Mg₇, but ishomogeneous, so is superior in workability, is superior in surfaceflatness, and has many other advantages. If desiring to particularlyimpart corrosion resistance to an amorphous phase plating layer, it issufficient to mix Zn₃Mg₇ in the plating layer.

A plating layer containing Zn₃Mg₇ has a superior sacrificialcorrosion-proofing ability with respect to steel sheet compared with a55% Al—Zn plating, Al-10% Si plating, etc.

To measure the sacrificial corrosion-proofing ability, it is sufficientto bend the hot dip plated steel sheet and find the corrosion resistanceof the worked part by a salt water spray test or cycle corrosion test.If an alloy plated steel sheet, the plating layer of the worked partcracks, so part of the steel sheet becomes exposed.

A 55% Al—Zn plated steel sheet, Al-10% Si plated steel sheet, etc. witha low sacrificial corrosion-proofing ability have red rust formed at theworked part immediately after the start of the test, but in hot dipMg—Zn plated steel sheet, the exposed part of the steel sheet of theworked part is immediately covered by Mg oxides, so the formation of redrust is greatly delayed.

An Mg—Zn amorphous plated steel material, Mg—Zn amorphous-phasecontaining plated steel material, and Zn₃Mg₇-containing plated steelmaterial all are hot dip Mg-based alloy plated steel materials havingnonequilibrium phases, so during the process of production, require atleast water cooling, high pressure mist cooling, or other cooling with arelatively large effect of cooling.

In particular, a large cooling rate is required for increasing theamount of the nonequilibrium phase superior in corrosion resistance.

Here, in actuality, there are at least two problems when producing anonequilibrium phase Mg—Zn hot dip plated steel material.

One is that when introducing a cooling facility with a large coolingeffect in a plating process, setting a cooling facility with a highcooling ability right after hot dip plating handling a high temperaturehot dip plating metal leads to a rise in costs.

The inventors studied the series of heat processes of reheating andrapidly cooling a plating layer (hereinafter referred to as“reheating/rapid cooling”) for the purpose of increasing the amount ofthe nonequilibrium phase contained in the plating layer using anequilibrium phase hot dip Mg—Zn alloy plating as a starting point.

As a result, the inventors discovered that when Mg, Zn, and Ca are in aspecific range of composition and applying reheating/rapid cooling ofspecific conditions to a plating layer, the alloying of the Zn in theplating layer and the Fe supplied from the steel material is suppressed.

Usually, if holding a plating layer containing Zn at 400° C. or more,the Zn in the plating layer and the Fe supplied from the steel materialreact to form a Γ-phase, δ-phase, or other intermetallic compound phase(that is, alloying occurs).

Hot dip galvannealed steel sheet (GA), widely used in the automobilefield, is Zn—Fe plated steel sheet deliberately utilizing thismetallurgical phenomenon to improve the weldability and the corrosionresistance after painting.

However, Mg and Ca are elements poor in reactivity with Fe and lower theactivity of Fe and Zn, so if Mg and/or Ca is present in the platingalloy in a certain concentration or more, intermetallic compounds of Znand Fe are hard to form during hot dip plating. Further, even if meltingagain after plating, intermetallic compounds of Zn and Fe are hard toform.

The range of composition enabling suppression of this alloying should bein the range of composition shown in FIG. 1. That is, it is possible tosuppress alloying if a Mg—Zn hot dip plating layer containing Zn: 15% ormore, Mg: 35% or more, and Ca: 15% or less.

Of course, when in the range of composition shown in FIG. 1, but outsidethe range of composition shown in FIG. 3 or FIG. 5, even in a region ofcomposition where a nonequilibrium phase is not much obtained, it ispossible to confirm that the amount of the nonequilibrium phase rises,though slightly, by confirming by DSC that the amount of exothermicpeaks due to the increasing of the amount of the nonequilibrium.

Alloying can be suppressed when heating the alloy plated steel materialfrom a temperature near the melting point of the plating bath (meltingpoint in range of composition shown in FIG. 1 of 580° C. or less), thatis, the melting point, to a temperature within (melting point+100° C.)and holding it in a short time (about 1 minute).

When holding an alloy plated steel material for a long period at atemperature near the melting point of the plating bath or when heatingto a much higher temperature than the melting point, even if thecomposition of the plating layer is a composition in the range ofcompositions shown in FIG. 1, alloying of Zn and Fe can occur.

Even if making the plating layer thick, sometimes some Fe—Znintermetallic compounds are formed near the interface between theplating layer and steel sheet, but the Fe—Zn intermetallic compoundswill never grow and alloying will never progress during the heating andrise in temperature of the alloy plated steel sheet.

The Fe required for securing the adhesion of the plating layer is a fineamount of about 0.1% or so. Further, the Fe which can be contained inthe plating layer as a whole is about 3%, but this extent of amount ofFe almost never leads to alloying with Zn.

Alloying of Fe and Zn remarkably progresses when 10% or so of Fe iscontained in the plating layer. Under heat treatment heating from themelting point of the plating bath to a temperature within the (meltingpoint+100° C.) and holding there for a short time (about 1 minute), theactivity of Fe in the Mg falls and alloying of Fe and Zn does not occur.

The alloying of Fe and Zn is confirmed by detecting intermetalliccompounds using X-ray diffraction through the plating layer, or bydetecting intermetallic compounds using a scanning electron microscopewith an energy dispersive X-ray spectrometer (SEM-EDX) at thecross-section of the plating layer, etc.

Usually, a Zn—Fe alloy layer grows from the interface, so it is possibleto use an optical microscope to examine the plating layer-steel sheetinterface so as to easily confirm the existence of a Zn—Fe alloy layer.

To confirm the suppression of Zn and Fe alloy, it is effective toinvestigate the ingredients of the plating layer before and afterreheating. Usually, if the Fe contained in the plating layer is lessthan 0.5%, Zn—Fe intermetallic compounds will almost never be observed.

If Fe becomes 0.5% or more, some Fe—Zn intermetallic compounds will beproduced near the interface of the plating layer and steel sheet, but ifreheating to a suitable temperature, the intermetallic compounds willnot grow during the rise in temperature and alloying will not progress.

The ingredients in the plating layer may be analyzed by preparing about50 ml of a plating layer dissolving solution by 10% hydrochloric acidetc. plus an inhibitor, using this plating layer dissolving solution topickle only the plating layer, and analyzing the ingredients in thedissolving solution after pickling by an ICP mass spectrometryapparatus.

The advantage of reheating/rapid cooling lies in increasing the amountof the nonequilibrium phase in addition to the independence of the rapidcooling process. When producing a steel material provided with a Mg—Znhot dip plating layer containing a nonequilibrium phase, it is necessaryto gas wipe the surface after plating to adjust the plating layer to thetarget thickness, then rapidly cool it.

At the time of gas wiping performed right after plating, if the drop intemperature of the plating layer is large, the plating layercrystallizes before the rapid cooling and, after the rapid cooling, nononequilibrium phase of the amorphous phase is produced and the platinglayer ends up becoming the same as the plating layer produced underequilibrium conditions.

To obtain an amorphous phase or other nonequilibrium phase, it isimportant to cool the plating layer by a sufficiently large cooling ratefrom a temperature right above the melting point of the plating bath.

The temperature of the plating bath is usually set to a temperature 10to 100° C. higher than the melting point of the plating alloy for thepurpose of improving the adhesion of the plating layer and steelmaterial, holding the plating bath stable, etc.

However, making the temperature of the plating bath further higher forthe above purpose is not preferable in terms of costs. Further, problemssuch as the increase of the production of dross and ignition of Mgcharacteristic of Mg-based alloy platings are also caused.

If the temperature of the plating bath becomes further higher, the steelmaterial temperature rises and the cooling rate at the time of coolingfalls. In particular, when using water cooling for cooling, the amountof production of steam due to the heat capacity of the steel materialincreases, the cooling rate further falls, and the amount of thenonequilibrium phase becomes smaller.

However, with the hot dip Mg—Zn plating layer of the present invention,even if the amount of the nonequilibrium phase is small, it is possibleto use-reheating to heat to right above the melting point of the platingbath, melt the plating layer again one time to eliminate the crystalphase or equilibrium phase, then rapidly cool it to cause the formationof an amorphous phase or other nonequilibrium phase so as to increasethe amount of the nonequilibrium phase.

That is, if a hot dip Mg-based alloy plating layer of the range ofcomposition of the present invention, it is possible to suppressalloying of Zn and Fe, so it is possible to reheat/rapidly cool theplating layer without alloying.

The reheating/rapid cooling is cooling for rapidly cooling from thetemperature right above the melting point of the plating bath, so it ispossible to cool down to the glass transition temperature in a shorttime. This is a cooling pattern suitable for obtaining an amorphous hotdip plated steel material.

Note that the conditions at the time of reheating govern the progress ofalloying between Zn and Fe. If the reheating temperature is too high orthe holding time is long—even at a temperature immediately above themelting point of the plating bath, even plating of the range ofcomposition of the present invention may alloy.

The inventors studied the reheating conditions and as a result learnedthat a temperature 10 to 100° C. higher than the melting point of theplating bath is suitable as the holding temperature and that the holdingtime is preferably within 1 minute.

Further, to suppress alloying of Fe and Zn, it is preferable to hold theplating layer at 500° C. or less.

When this condition is not met, that is, when excessively raising thetemperature, the diffusion of Fe is unnecessarily activated and alloyingis facilitated. The rate of temperature rise at the time of reheating isnot particularly limited, but the rate of temperature rise is preferablyslow so as to make the temperature of the plating layer as a wholeconstant and further to prevent overheating due to a rapid temperaturerise.

In the hot dip Mg—Zn alloy plating layer, it is difficult to secureadhesion between the plating layer and steel sheet due to the poorreactivity of Mg and Fe.

In particular, when the Mg concentration is high, “non-plating defects”easily occur. Securing adhesion with the steel sheet also becomes moredifficult, but it is possible to use the preplating method to suppress“non-plating defects” and easily secure adhesion with the steel sheet.

A preplating layer must have “wettability” with the plating alloy. Theinventors investigated the wettability with an Mg-based plating alloyfor various alloy elements for securing adhesion between the platinglayer and steel sheet.

As a result, it was learned that Cr, Co, Ni, Cu, Ag, and/or Sn aresuitable as preplating metal. The preplating layer may also be a platinglayer of an alloy combining a selection of two or more of these metals.

This metal preplating layer is preferably formed by electroplating orelectroless plating. The thickness of the preplating layer should be 0.1to 1 μm (deposition amount of 1 to 10 g/m²).

After plating by ordinary Mg—Zn hot dip plating conditions (bathtemperature of 350 to 600° C.), the preplating layer sometimes remains.

If the preplating layer is too thin, the effect of suppression ofnon-plating defects and the effect of securing adhesion cannot beexpected.

After the plating, the elements forming the preplating layer diffuseinside the plating layer and are included in the plating layer inamounts of 1% or so. The amounts of elements diffused from thepreplating layer are very small and form a substitution type solidsolution in the plating layer.

The “non-plating defects” can be easily confirmed visually. The numberof the “non-plating defects” present in a certain range from the centerof the plated steel sheet is confirmed visually and the extent of“non-plating defects” judged by the number per unit area.

Note that the number of the “non-plating defects” of the steel sheetsurface changes with the immersion speed of the steel sheet in theplating bath, so when confirming the effect of the preplating, it ispreferable to make the immersion speed of the steel sheet in the platingbath constant.

The material of the steel material forming the substrate of the presentinvention steel material is not particularly limited. Al-killed steel,ultralow carbon steel, high carbon steel, various high-tensile steel, Nisteel, Cr steel, Ni—Cr steel, etc. can be used.

The steelmaking method, strength of the steel, hot rolling method,pickling method, cold rolling method, etc. are also not particularlylimited.

For the plating method, the Sendimir method; preplating method, two-stephot dipping method, flux method, etc. may be used. As the preplatingbefore the Mg—Zn alloy plating of the present invention, Ni plating,Sn—Zn plating, etc. may be used.

The steel material provided with an Mg—Zn alloy plating layer of thepresent invention is preferably produced by a vacuum or inert gasatmosphere. As the preplating before the Mg—Zn alloy plating of thepresent invention and the first step dipping in two-step hot dipping, Niplating, Zn plating, Sn—Zn plating, etc. may be used.

The alloy used for the plating bath may be produced in advance withoutworrying about the ignition point of Mg if melting Mg and Zn mixed in apredetermined ratio in a “crucible” with an inside replaced with aninert gas etc.

There is also the method of utilizing commercially availablenon-combustible Mg. In this case, it is sufficient to mix predeterminedamounts of the non-combustible Mg and Zn and melt them near 600° C.However, non-combustible Mg sometimes contains Al or Ca. In this case,the plating bath will also contain Al or Ca.

By having the plating bath contain Mg in a high concentration, it ispossible to suppress the formation of a Zn—Fe alloy layer. For thisreason, it is not necessary to add Al to the plating bath for thepurpose of suppressing formation of a Zn—Fe alloy layer.

The formation of a Zn—Fe alloy layer with poor plastic deformabilityalso causes powdering, flaking, and other peeling of the plating layerdue to working after plating. An Mg-based alloy plating layer containingMg in a high concentration of the present invention has the advantage ofnot causing peeling of the plating layer.

Regarding the addition of Fe, Cr, Cu, Ag, Ni, Ti, Zr, Mo, Si, and/or Nb,if adding small amounts of up to a total of around 0.1%, it is possibleto introduce them into the plating bath by adding metal powders to theplating bath and holding the bath there in an inert atmosphere at around600° C. for a long period of time.

When adding the above metals in a high concentration, an atmosphericfurnace etc. is used to prepare an alloy of the added metals and Zn orMg and this alloy is added to the plating bath. In the preparation ofthis added alloy as well, since Zn has a low boiling point, it ispreferably melted at 900° C. or less.

Regarding the addition of Al, Ca, Y, and/or La, if adding them up to atotal of around 5%, it is possible to introduce them into the platingbath by adding metal powders to the plating bath and holding the baththere in an inert atmosphere at around 600° C. for a long period oftime.

When adding the above metals over 5%, an atmospheric furnace etc is usedto prepare an alloy of the added metals and Zn or Mg and this alloy isadded to the plating bath.

If an Mg—Zn alloy plating where Ca, Y, La, etc. are added to raise theamorphous phase forming ability, it is possible to easily obtain asingle phase of an amorphous phase by hot dip plating, then cooling theplating layer by for example mist cooling etc. from a close distancewhere a cooling rate of about 10 to 1000° C./sec is obtained at thesurface layer of the plating.

In another Mg—Zn system where Ca, Y, La, etc. are not added and theamorphous phase forming ability is small, it is possible to obtain acooling rate of about 1000 to 5000° C./sec at the surface layer of theplating and to produce an amorphous hot dip plated steel sheet comprisedof mixed phases of fine crystals and an amorphous phase by water coolingplated steel sheet after hot dip plating or immersing the plated steelsheet in water right after hot dip plating.

Furthermore, to increase the cooling rate, there are the methods ofmaking the substrate thinner, making the plating layer thinner, using asub-zero alcohol based refrigerant, etc.

The volume percentage of the amorphous phase depends on the amorphousphase forming ability based on the plating composition. If the platingcomposition of the present invention, it is possible to obtain a platinglayer containing an amorphous phase of at least 5 vol % by making thetemperature of the plating layer substantially the same as the meltingpoint of the plating bath and immersing it in 0° C. water.

In a system of ingredients to which Ca, Y, La, etc. are not added andwith a small amorphous phase forming ability, in order to obtain anamorphous phase, it is possible to make the amount of plating depositionsufficiently small (for example, making the plating thickness 6 μm orless), make the temperature of the plating layer right before immersionin water substantially the same as the melting point, immerse it in 0°C. water, and sufficiently increase the cooling rate of the platinglayer to thereby obtain a plating layer containing an amorphous phase of5 vol % or more.

Conversely, in a system of ingredients in which Ca, Y, La, etc. areadded, the amorphous phase forming ability is high, so even if thetemperature right before water immersion is somewhat higher than themelting point of the plating bath, it is possible to obtain a platinglayer comprised of a single phase of an amorphous phase by justimmersion in ordinary temperature water.

When desiring to deliberately reduce the volume percentage of theamorphous phase, mist cooling is used or the temperature immediatelybefore water immersion is raised.

The formation of an amorphous phase can be confirmed by a halo patternbeing obtained in the X-ray diffraction pattern of the plating layer. Ifa single amorphous phase, only a halo pattern (when the plating layer isthin, sometimes the Fe diffraction peak of the steel material of thesubstrate is detected) is obtained.

When the amorphous phase and crystal phase are mixed, if the amorphousphase volume percentage is low, it is possible to use a differentialscanning calorimeter to detect the exothermic peak when the amorphousphase crystallizes during the rise in temperature and thereby confirmthe presence of the amorphous phase in the plating layer.

To find the volume percentage of the amorphous phase, the plated steelmaterial is cut, the cross-section is polished and etched, and theplating layer of the surface is observed by an optical microscope.

At the part of the amorphous phase, no structure is observed due toetching, but at the part of the crystal phase, a structure due tocrystal grain boundaries, sub-grain boundaries, precipitates, etc. isobserved.

Due to this, it is possible to clearly differentiate the region of theamorphous phase part and the region of the crystal phase part, so it ispossible to calculate the volume rate by the line segment method orimage analysis.

When the structure is overly fine and measurement by an opticalmicroscope is difficult, a thin piece is obtained from the cross-sectionof the plating layer and examined under a transmission electronmicroscope.

In the case of a transmission electron microscope, it is possible toconfirm an amorphous structure by a halo pattern of an electron beamdiffraction pattern in a region where no structure can be observed.

In observation by an optical microscope, when texture is not observedover the entire surface or when, even if there are parts where texturecannot be observed, there is a suspicion of being coarse, strain freecrystal grains, it is preferable to further obtain a thin piece for anelectron microscope and confirm that there are no diffraction spots onthe electron beam diffraction pattern and that a halo pattern isobserved so as to confirm that this is an amorphous phase.

With both an optical microscope and an electron microscope, it ispreferable to find the area rate in 10 or more different fields bycomputer image processing, find the average of the found area rates, anduse the result as the volume rate.

For the detection of the Zn₃Mg₇ in the plating layer, the general X-raydiffraction method is effective. For example, an X-ray diffractionapparatus using Cu—Kα rays is used to measure the diffraction patternand judge presence by the presence of a Zn₃Mg₇ diffraction peak.

In this case, for identification of the Zn₃Mg₇ by the X-ray diffractionpattern, it is preferable to use the diffraction peak of 2θ=10 to 30°.This is because if over 30°, it overlaps with the strongest line of theMg diffraction peak.

Further, when the amount of the Zn₃Mg₇ phase is small, judgment byTEM-EDX is also effective. The characteristic X-ray spectrum obtainedfrom a specific crystal phase may be used to identify the Zn₃Mg₇.

EXAMPLES

Next, examples of the present invention will be explained. Theconditions of the examples are examples of conditions employed forconfirming the workability and effects of the present invention. Thepresent invention is not limited to these examples of conditions. Thepresent invention can employ various conditions so long as not departingfrom the gist of the present invention and achieving the object of thepresent invention.

Example 1

A surface treated steel material was prepared using a bath of each ofthe plating compositions shown in Tables 1 to 6 and cold rolled steelsheet of a thickness of 0.8 mm, equal angle steel of a thickness of 10mm and a length per side of 10 cm, or hot rolled steel sheet of athickness of 10 mm as a substrate.

Mg, Zn, and other necessary ingredient elements were adjusted to apredetermined composition, then a high frequency induction furnace wasused to melt them in an Ar atmosphere to obtain an Mg—Zn alloy.

Scraps were taken from the prepared alloy and dissolved by acid. Thesolution was assayed by ICP (inductively coupled plasma) massspectrometry to confirm that the prepared alloy matched with thecomposition shown in Tables 1 to 6. This alloy was used as the platingbath.

Cold rolled steel sheet (thickness of 0.8 mm) was cut to 10 cm×10 cm toobtain a test piece. This test piece was plated by a batch type hot dipplating test apparatus made by Rhesca. The bath temperature of theplating bath was made 500° C. Air wiping was used to adjust the amountof deposition, then nitrogen gas was used to cool the plating down toordinary temperature.

For preparation of an amorphous hot dip plated steel sheet containing anamorphous phase in a volume percentage of 5% or more, the plated steelsheet was immersed in 0° C. water after hot dip plating.

For preparation of an amorphous hot dip plated steel sheet containing anamorphous phase in a volume percentage of less than 5%, the plated steelsheet was cooled by spraying high pressure mist from a close distance.

The equal angle steel was cut to 10 cm in the long direction, while thehot rolled steel sheet was cut to a square of 10 cm×10 cm to obtain atest piece.

First, this cut piece was hot dip plated in a Zn bath using the fluxmethod using a crucible furnace to give an amount of deposition of about100 g/m², then was immersed in a Zn—Mg alloy bath of the presentinvention composition and, as needed, cooled by immersion in 0° C.water.

The plating adhesion was evaluated, for a cold rolled steel sheet, bybending a plated test piece by 180° with the plating layer at theoutside and subjecting it to an 8T bending test. After this, the platinglayer of the bent part was peeled off by adhesive tape, thecross-section of the bent part was examined under an optical microscope,and the rate of deposition of the plating layer at the outercircumference of the cross-section of the bent part was found.

A residual rate of the plating layer after the test of 50 to 100% wasevaluated as “G (good)” and less than 50% as “P (poor)”. No platinglayer was indicated by “−”.

For hot rolled steel sheet and equal angle steel, the cross-section ofthe bent part was examined under an optical microscope and the rate ofdeposition of the plating layer at the outer circumference of thecross-section of the bent part was found. A test piece with a rate ofdeposition of the plating layer of 50 to 100% was evaluated as “G(good)” and less than 50% as “P (poor)”. No deposition of plating layerwas indicated by “−”.

The formation of an amorphous phase of the surface of the plating layeris judged by using an X-ray diffraction apparatus using Cu—Kα rays tomeasure the diffraction pattern and judging the presence of a halopattern.

When the amorphous phase and crystal phase are mixed, if the volumepercentage of the amorphous phase is low, a differential scanningcalorimeter can be used to detect the exothermic peak when crystallizingfrom the amorphous phase during the rise in temperature so as to confirmthe presence of the amorphous phase.

To quantitatively find the volume percentage of the amorphous phase fora plated steel sheet judged to have an amorphous phase, the plated steelsheet was cut, its cross-section was polished and etched, then theplating layer of the surface was examined under an optical microscope(×1000).

The area rate of the amorphous phase was found for 10 or more differentfields by computer image processing and the area rates found wereaveraged to obtain the volume rate.

The corrosion resistance of the plated steel sheet was evaluated byapplying the method based on an automotive standard (JASO M609-91, 8hours/cycle, wetting/drying time ratio 50%) for 21 cycles. For the saltwater, 0.5% saline was used. The corrosion resistance was evaluated bythe corrosion loss calculated from the corrosion loss and density afterthe tests.

A corrosion loss of less than 0.5 μm was evaluated as “VG (very good)”,0.5 to 1 μm as “G (good)”, 1 to 2 μm as “SG (somewhat good)”, 2 to 3 μmas “F (fair)”, and 3 μm or more as “P (poor)”. In Tables 1 to 6, testpieces with plating adhesions evaluated as “P” were not evaluated forcorrosion resistance, so “−” are shown.

TABLE 1 Amor- Ad- phous Steel Plating composition (atm %) he- Corrosionpercentage No. material Phase Zn Mg Al Ca La Y Si Ti Cr Cu Fe Ni Zr NbMo Ag sion resistance (%) Present 1-1  Cold C 15 85 G F 0 in- 1-2 rolled C 20 80 G F 0 vention 1-3  steel C 25 75 G F 0 1-4  sheet C 30 70G F 0 1-5  C 35 65 G F 0 1-6  C 40 60 G F 0 1-7  C 44 56 G F 0 1-8  C 3065 5 G SG 0 1-9  C 30 67 3 G SG 0 1-10 C 30 67 3 G SG 0 1-11 C 30 68 1 1G SG 0 1-12 C 30 69 1 G SG 0 1-13 C 30 69 1 G SG 0 1-14 C 30 69.5 0.5 GSG 0 1-15 C 30 69.97 0.03 G SG 0 1-16 C 30 69.97 0.03 G SG 0 1-17 C 3069.8 0.2 G SG 0 1-18 C 30 69.97 0.03 G SG 0 * Notations in “Phase”column means the following: C: Plating layer comprised of only crystalphase and A: Plating layer including 5% or more amorphous phase.

TABLE 2 Amorphous Steel Plating composition (atm %) Adhe- Corrosionpercentage No. material Phase Zn Mg Al Ca La Y Si Ti Cr Cu Fe Ni Zr NbMo Ag sion resistance (%) Present 2-1  Cold C 30 65 5 G SG 0 invention2-2  rolled C 25 65 10 G SG 0 2-3  steel C 15 70 15 G SG 0 2-4  sheet C50 48 2 G SG 0 2-5  C 15 80 5 G SG 0 2-6  C 20 75 5 G SG 0 2-7  C 25 705 G SG 0 2-8  C 30 65 5 G SG 0 2-9  C 35 60 5 G SG 0 2-10 C 40 55 5 G SG0 2-11 C 45 50 5 G SG 0 2-12 C 50 45 5 G SG 0 2-13 C 55 40 5 G SG 0 2-14C 59 36 5 G SG 0 2-15 C 30 60 10 G SG 0 2-16 C 20 70 10 G SG 0 2-17 C 4050 10 G SG 0 2-18 C 25 60 15 G SG 0 * Notations in “Phase” column meansthe following: C: Plating layer comprised of only Crystal phase and A:Plating layer including 5% or more amorphous phase.

TABLE 3 Amor- steel Ad- phous ma- Plating composition (atm %) he-Corrosion percentage No. terial Phase Zn Mg Al Ca La Y Si Ti Cr Cu Fe NiZr Nb Mo Ag sion resistance (%) Present 3-1  Cold C 40 45 15 G SG 0 in-3-2  rolled C 25 70 5 G SG 0 vention 3-3  steel C 30 60 10 G SG 0 3-4 sheet C 25 60 5 10 G SG 0 3-5  C 25 60 5 10 G SG 0 3-6  C 25 70 5 G SG 03-7  C 30 60 10 G SG 0 3-8  C 25 60 5 10 G SG 0 3-9  C 25 65 5 5 G G 03-10 C 27 65 5 3 G G 0 3-11 C 27 65 5 3 G G 0 3-12 C 29 65 5 1 G G 03-13 C 29 65 5 1 G G 0 3-14 C 29.5 65 5 0.5 G G 0 3-15 C 29.97 65 5 0.03G G 0 3-16 C 29.97 65 5 0.03 G G 0 3-17 C 29.8 65 5 0.2 G G 0 3-18 C29.97 65 5 0.03 G G 0 * Notations in “Phase” column means the following:C: Plating layer comprised of only crystal phase and A: Plating layerincluding 5% or more amorphous phase.

TABLE 4 Amorphous Steel Plating composition (atm %) Corrosion percentageNo. material Phase Zn Mg Al Ca La Y Si Ti Cr Cu Fe Ni Zr Nb Mo AgAdhesion resistance (%) Present 4-1  Cold A 25 75 G SG 5 invention 4-2 rolled A 30 70 G SG 5 4-3  steel A 15 80 5 G G 10 4-4  sheet A 20 75 5 GVG 90 4-5  A 25 70 5 G VG 100 4-6  A 30 65 5 G VG 100 4-7  A 35 60 5 GVG 90 4-8  A 30 60 10 G G 80 4-9  A 20 70 10 G G 50 4-10 A 25 60 15 G G45 4-11 A 25 70 5 G G 80 4-12 A 30 60 10 G G 70 4-13 A 25 60 5 10 G G 204-14 A 25 70 5 G G 70 4-15 A 30 60 10 G G 20 4-16 A 25 65 5 5 G VG 704-17 A 27 65 5 3 G VG 80 4-18 A 27 65 5 3 G VG 80 * Notations in “Phase”column means the following: C: Plating layer comprised of only crystalphase and A: Plating layer including 5% or more amorphous phase.

TABLE 5 Amor- Ad- phous Steel Plating composition (atm %) he- Corrosionpercentage No. material Phase Zn Mg Al Ca La Y Si Ti Cr Cu Fe Ni Zr NbMo Ag sion resistance (%) Present 5-1 Cold A 29 65 5 1 G VG 100 in- 5-2rolled A 29 65 5 1 G VG 100 vention 5-3 steel A 29.5 65 5 0.5 G VG 1005-4 sheet A 29.97 65 5 0.03 G VG 100 5-5 A 29.97 65 5 0.03 G VG 100 5-6A 29.8 65 5 0.2 G VG 100 5-7 A 29.97 65 5 0.03 G VG 100 5-8 Hot C 25 705 G SG 0 5-9 rolled A 25 70 5 G VG 40 steel sheet  5-10 Equal C 25 70 5G SG 0  5-11 angle A 25 70 5 G VG 40 steel * Notations in “Phase” columnmeans the following: C: Plating layer comprised of only crystal phaseand A: Plating layer including amorphous phase.

TABLE 6 Amorphous Steel Plating composition (atm %) Corrosion percentageNo. material Phase Zn Mg Al Ca La Y Si Ti Cr Cu Fe Ni Zr Nb Mo AgAdhesion resistance (%) Comp. 6-1 Cold C 100 G P 0 ex. 6-2 rolled C 1387 — — 0 6-3 steel C 45 55 P — 0 6-4 sheet C 50 49 1 P — 0 6-5 C 60 35 5P — 0 6-6 C 40 40 20 P — 0 6-7 C 20 60 20 P — 0 6-8 C 30 50 20 P — 0 6-9C 20 60 20 P — 0  6-10 C 20 67 5 8 P — 0 Inv.  6-11 C 20 75 5 G SG 3 ex. 6-12 C 25 70 5 G SG 4  6-13 C 30 65 5 G SG 3 * Notations in “Phase”column means the following: C: Plating layer comprised of only crystalphase or plating layer including less than 5% amorphous phase and A:Plating layer including 5% or more amorphous phase.

As shown in Table 1 to 6, the hot dip Mg—Zn plated steel material of thepresent invention maintains sufficient performance in plating adhesion.The corrosion resistances of the steels of the present invention are allbetter than that of the hot dip Zn plated steel sheet (No. 6-1).

The plated steel materials containing Si, Ti, Cr, Cu, Fe, Ni, Zr, Nb,Mo, Ag, Al, Ca, Y, and/or La in the plating layers are further superiorin corrosion resistance. Among these, the plated steel materialsprovided with plating layers containing the above elements andcontaining amorphous phases are particularly superior in corrosionresistance.

Table 7 and Table 8 show the results of evaluation of the corrosionresistance comparing amorphous hot dip plated steel sheet and platedsteel sheet of only crystal phases. As clear from Table 7 and Table 8,plated steel sheet having amorphous phases in the case of the sameingredients are superior in the point of corrosion resistance.

TABLE 7 Amorphous Plating composition (atm %) percentage Corrosion No.Phase Zn Mg Al Ca La Y Si Ti Cr Cu Fe Ni Zr Nb Mo Ag (%) resistance 1-3C 25 75 0 F 4-1 A 25 75 5 SG 1-4 C 30 70 0 F 4-2 A 30 70 5 SG 2-5 C 1580 5 0 SG 4-3 A 15 80 5 10 G 2-6 C 20 75 5 0 SG 4-4 A 20 75 5 90 VG 2-7C 25 70 5 0 SG 4-5 A 25 70 5 100 VG 2-8 C 30 65 5 0 SG 4-6 A 30 65 5 100VG 2-9 C 35 60 5 0 SG 4-7 A 35 60 5 90 VG  2-15 C 30 60 10 0 SG 4-8 A 3060 10 80 G  2-16 C 20 70 10 0 SG 4-9 A 20 70 10 50 G  2-18 C 25 60 15 0SG  4-10 A 25 60 15 45 G 3-2 C 25 70 5 80 SG  4-11 A 25 70 5 80 G 3-3 C30 60 10 0 SG  4-12 A 30 60 10 70 G * Notations in “Phase” column meansthe following: C: Plating layer comprised of only crystal phase and A:Plating layer including 5% or more amorphous phase.

TABLE 8 Plating composition (atm %) Amorphous Corrosion No. Phase Zn MgAl Ca La Y Si Ti Cr Cu Fe Ni Zr Nb Mo Ag percentage (%) resistance 3-5 C 25 60 5 10 0 SG 4-13 A 25 60 5 10 20 G 3-6  C 25 70 5 0 SG 4-14 A 2570 5 70 G 3-7  C 30 60 10 0 SG 4-15 A 30 60 10 20 G 3-9  C 25 65 5 5 0 G4-16 A 25 65 5 5 70 VG 3-10 C 27 65 5 3 0 G 4-17 A 27 65 5 3 80 VG 3-11C 27 65 5 3 0 G 4-18 A 27 65 5 3 80 VG 3-12 C 29 65 5 1 0 G 5-1  A 29 655 1 100 VG 3-13 C 29 65 5 1 0 G 5-2  A 29 65 5 1 100 VG 3-14 C 29.5 65 50.5 0 G 5-3  A 30 65 5 0.5 100 VG 3-15 C 29.97 65 5 0.03 0 G 5-4  A 3065 5 0.03 100 VG 3-16 C 29.97 65 5 0.03 0 G 5-5  A 30 65 5 0.03 100 VG3-17 C 29.8 65 5 0.2 0 G 5-6  A 30 65 5 0.2 100 VG 3-18 C 29.97 65 50.03 0 G 5-7  A 30 65 5 0.03 100 VG * Notations in “Phase” column meansthe following: C: Plating layer comprised of only crystal phase and A:Plating layer including 5% or more amorphous phase.

FIG. 6 shows the cross-section of the Plated Steel Sheet No. 2-7 (amountof deposition 20 g/m²) provided with a Mg-25 atm % Zn-5 atm % Ca platinglayer (crystal phase).

As can be judged from FIG. 6, there are no cracks or peeling at theinterface of the steel sheet 5 and Mg-25 atm % Zn-5 atm % Ca platinglayer (crystal phase) 4. It will be understood that at the steel sheet 5and Mg-25 atm % Zn-5 atm % Ca plating layer (crystal phase) 4, a goodadhesion is obtained and that an Mg—Zn alloy containing Mg in a highconcentration can be hot dip plated on steel sheet.

FIG. 7 shows the cross-section of the Plated Steel Sheet No. 4-5 (amountof deposition 20 g/m²) obtained by cooling Mg by immersion in water andforming an Mg-25 atm % Zn-5 atm % Ca plating layer (amorphous phase) 6on the steel sheet 5.

FIG. 8 shows the X-ray-diffraction pattern of this plating layer. By thedetection of a halo pattern in the X-ray diffraction pattern, it islearned that the Mg-25 atm % Zn-5 atm % Ca plating layer (amorphousphase) 6 shown in FIG. 7 is an amorphous phase.

FIG. 9 shows an FE-TEM image (bright field image) near the interface ofthe plated steel sheet comprised of the steel sheet 9 formed with anMg-25 atm % Zn-5 atm % Ca plating layer (amorphous phase) 8.

FIG. 10 shows the result of elemental analysis by EDX at the cross pointof FE-TEMA of FIG. 9. It will be understood that Fe is diffused insidethe plating layer.

FIG. 11 shows an electron beam diffraction pattern at a cross point ofthe FE-TEM image of FIG. 9. A halo pattern is detected. It will beunderstood that the Mg-25 atm % Zn-5 atm % Ca plating layer (amorphousphase) 8 shown in FIG. 9 is an amorphous phase even near the interfaceand is a single amorphous phase.

Example 2

A surface treated steel material was prepared using a bath of each ofthe plating compositions shown in Table 9 and cold rolled steel sheet ofa thickness of 0.8 mm as a substrate. The substrate was pretreated forpreplating by alkali degreasing and pickling.

The Ni preplating layer was formed by dipping a test piece in a 30° C.aqueous solution containing nickel sulfate: 125 g/l, ammonium citrate:135 g/l, and sodium hypophosphate: 110 g/l mixed together and adjustedby sodium hydroxide to pH10.

The Co preplating layer was formed by dipping a test piece in a 90° C.aqueous solution containing cobalt sulfate: 15 g/l, sodiumhypophosphate: 21 g/l, sodium citrate: 60 g/l, and ammonium sulfate: 65g/l mixed together and adjusted by aqueous ammonium to pH10.

The Cu preplating layer was fabricated by dipping a test piece in a 25°C. aqueous solution containing copper sulfate: 2 g/l and sulfuric acid:30 g/l mixed together.

The Cu—Sn preplating layer was fabricated by dipping a test piece in a25° C. aqueous solution containing copper chloride: 3.2 g/l, stannouschloride: 5.0 g/l, and hydrochloric acid: 8 g/l mixed together.

The Ag preplating layer was fabricated by electroplating in a solutionof silver cyanide 2 g/l and potassium cyanide 80 g/l mixed together anda temperature of 30° C. by a current density of 2 A/dm².

The Cr preplating layer was fabricated by electroplating in a solutionof anhydrous chromic acid 250 g/l and sulfuric acid 2.5 g/l mixedtogether and a temperature of 50° C. by a current density of 20 A/dm².

Using these plating baths, the dipping times were adjusted to make thedeposition amounts 1 to 5 g/m². The amount of deposition of eachpreplating was determined by dissolving the preplating in nitric acidetc., quantitatively analyzing the solution by ICP (inductively coupledplasma) mass spectrometry, and converting the amounts of dissolvedelements to the amount of deposition.

Mg, Zn, and other necessary elements were prepared into a predeterminedcomposition, then a high frequency induction furnace was used to melt itin an Ar atmosphere to obtain an Mg—Zn alloy. Scraps were obtained fromthe prepared alloy and dissolved in an acid. The solution was thenassayed by ICP (inductively coupled plasma) mass spectrometry to confirmthat the prepared alloy matched the composition shown in Table 9. Thisalloy was used as the plating bath.

Cold rolled steel sheet (thickness 0.8 mm) was cut to 10 cm×20 cm foruse as a test piece. This test piece was plated by a batch type hot dipplating test apparatus made by Rhesca.

For the cold rolled steel sheet, one which was preplated and one in theoriginal state were used. Each was hot dip plated. The bath temperatureof the plating bath was made 400 to 600° C. The amount of deposition wasadjusted by air wiping.

The dipping rate of the steel sheet in the plating bath was made 500mm/sec. The sample was dipped for 3 second, adjusted in amount ofdeposition by air wiping, then immediately reheated and water cooled bywater cooling, air cooling, or a later explained technique.

After dipping, the number of “non-plating defects” (visually discernable1 mm or larger “non-plating defects”) at the center part of the platedsteel sheet (5 cm×10 cm) was counted and converted to the number of“non-plating defects” per 50 cm².

For each sample, the average was found for n=10. A number of“non-plating defects” of one or less was evaluated as “VG (very good)”,1 to 3 as “G (good)”, 5 to 10 or more as “F (fair)”, and 10 or more as“P (poor)”.

The diffraction pattern of the surface forming phase at the center part(20 mm×20 mm) of the prepared plated steel sheet was measured by anX-ray diffraction apparatus using Cu—Kα rays.

Using X-ray diffraction, the surface forming phase was identified. Onewhere a halo pattern was detected was evaluated as “G (good)”, while onewhere it could not obtained or where mixture of a crystal phase madejudgment difficult was evaluated as “F (fair)”.

Further, a test piece with a diffraction peak of a high temperaturestable phase Zn₃Mg₇ detected was evaluated as “E (excellent)”.“Detection of a peak” means an X-ray intensity ratio (ratio ofdiffraction peak intensity of Zn₃Mg₇ (excluding diffraction peak ofplane spacing of 0.233 nm) in the sum of all diffraction peakintensities appearing at diffraction plane intervals of 0.1089 to 1.766nm, that is, diffraction angles 2θ of 5 to 90° in case of diffractionmeasurement by Cu—Kα rays using an X-ray tube with Cu target for theX-ray source (however, excluding diffraction peak of plane spacing of0.233 nm) of 10% or more.

Further, a halo pattern was evaluated as “G (good)”, while observationof a diffraction peak of Zn₃Mg₇ as well was evaluated as “GE(good-excellent)”. FIG. 12 shows an X-ray diffraction pattern of No. 16in Table 9. This is an example of observation of both a halo pattern andZn₃Mg₇.

For the reheating and water cooling, after the plating, the amount ofdeposition was adjusted by air wiping, then the test pieces were allowedto cool to ordinary temperature. After being allowed to stand atordinary temperature, the test pieces were reheated to raise them intemperature to the hot dip plating bath temperature and held at thistemperature for 10 seconds, then were water cooled.

The corrosion resistance of the plated steel sheet was evaluated byapplying the method based on an auto standard (JASO M609-91, 8hours/cycle, wetting/drying time ratio 50%) for 21 cycles. For the saltwater, 0.5% saline was used. The corrosion resistance was evaluated bythe corrosion loss calculated from the corrosion loss and density afterthe tests.

A corrosion loss of less than 0.5 μm was evaluated as “VG (very good)”,0.5 to 1 μm as “G (good)”, 1 to 2 μm as “SG (somewhat good)”, 2 to 3 μmas “F (fair)”, and 3 μm or more as “P (poor)”.

FIG. 13 shows the X-ray diffraction pattern of Mg-27 atm % Zn-1 atm %Ca-6 atm % Al of No. 3 in Table 9. From the X-ray diffraction pattern,only the diffraction line of Zn₃Mg₇ could be obtained. Ca and Al arebelieved to form substitution type solid solutions and exist in thoseforms.

FIG. 14 shows the X-ray diffraction patterns of the plated steel sheetsurface forming phases of No. 3 and No. 6 to No. 8 in Table 9.

10 shows the X-ray diffraction pattern of an Mg-27 atm % Zn-1 atm % Ca-6atm % Al plating layer (No. 3), 11 shows the X-ray diffraction patternof an Mg-27 atm % Zn-1 atm % Ca-8 atm % Al plating layer (No. 6), 12shows the X-ray diffraction pattern of an Mg-27 atm % Zn-1 atm % Ca-10atm % Al plating layer (No. 7), and 13 shows the X-ray diffractionpattern of an Mg-27 atm % Zn-1 atm % Ca-13 atm % Al plating layer (No.8).

From the figure, it will be understood that in No. 3, the plating layeris a single Zn₃Mg₇ phase. As the Al concentration becomes higher, theamount of the Zn₃Mg₇ phase becomes smaller. In No. 8, it will beunderstood that the Zn₃Mg₇ almost completely disappears.

TABLE 9 Plating Non- ingredients Plating plating Plating post (atm %)bath Plating defects Corrosion Class No. Steel Preplating treatment MgZn Ca Al temp. XRD deposition (No.) resistance Invention 1 Cold Cu—SnWater cooling 73.7 25 0.8 0.5 450 E 25 VG G 2 rolled Cu—Sn Water cooling73 20 1 6 450 E 25 VG G 3 steel Cu—Sn Water cooling 66 27 1 6 450 E 25VG VG 4 sheet None Water cooling 450 E 25 F VG 5 Cu—Sn Air cooling 450 F25 VG SG 6 Cu—Sn Water cooling 64 27 1 8 450 E 25 VG VG 7 Cu—Sn Watercooling 62 27 1 10 450 E 25 VG G 8 Cu—Sn Water cooling 59 27 1 13 500 F25 VG SG 9 Cu—Sn Water cooling 63 30 1 6 450 E 25 VG VG 10 Cu—Sn Watercooling 58 35 1 6 500 E 25 VG G 11 Cu—Sn Water cooling 53 40 1 6 550 E25 VG G 12 Cu—Sn Water cooling 64 25 5 6 500 E 25 VG G 13 Ni Reheating &80 15 5 550 G 30 VG G water cooling 14 Ni Reheating & 75 20 5 500 G 30VG VG water cooling 15 Ni Reheating & 70 25 5 450 G 30 VG VG watercooling 16 Ni Reheating & 66 25 5 4 450 GE 30 VG VG water cooling 17None Water cooling 70 25 5 450 G 30 F VG 18 Ni Water cooling 450 G 30 VGVG 19 Cr Water cooling 450 G 30 G VG 20 Co Water cooling 450 G 30 G VG21 Cu Water cooling 450 G 30 G VG 22 Ag Water cooling 450 G 30 G VG 23Ni Reheating & 65 30 5 450 G 30 VG VG water cooling 24 Ni Reheating & 6230 5 3 450 GE 30 VG VG water cooling 25 Ni Reheating & 60 35 5 500 G 30VG VG water cooling 26 Ni Reheating & 55 40 1 4 500 E 30 VG G watercooling 27 Ni Reheating & 50 45 1 4 550 E 30 VG G water cooling 28 NiAir cooling 550 F 30 VG SG 29 Ni Reheating & 53.7 45 0.8 0.5 550 E 30 VGG water cooling 30 Ni Air cooling 550 F 30 VG SG 31 Ni Reheating & 53.545 1.5 550 F 30 VG G water cooling 32 Ni Air cooling 550 F 30 VG SG 33Ni Reheating & 45 50 5 550 F 30 VG G water cooling 34 Ni Reheating &47.5 50 2 0.5 550 F 30 VG G water cooling 35 Ni Reheating & 48.5 50 1.5550 F 30 VG G water cooling 36 Ni Reheating & 43.5 55 1.5 600 F 30 VG Gwater cooling 37 Ni Reheating & 40 55 5 550 F 30 VG G water cooling 38Ni Reheating & 36 59 5 600 F 30 VG G water cooling 39 Ni Reheating & 7020 10 500 G 30 VG G water cooling 40 Ni Reheating & 40 50 10 550 F 30 VGG water cooling

INDUSTRIAL APPLICABILITY

As explained above, the present invention (hot dip Mg—Zn alloy platedsteel material) enables production by an ordinary hot dip platingprocess and is superior in universality and economy.

Further, the hot dip Mg—Zn alloy plating layer of the present inventionkeeps down the concentration of Zn yet gives a corrosion resistancesuperior to that of a conventional hot dip Zn plating layer, socontributes to saving Zn resources.

Further, the hot dip Mg—Zn alloy plating layer of the present inventionis excellent in not only corrosion resistance, but also workability, sothe present invention can be widely utilized as structural members andfunctional members in the fields of automobiles, building materials, andhousehold electrical appliances.

Accordingly, the present invention contributes to the development of themanufacturing industries by the increase in life of structural partsused in the automobile, building material, and household electricalappliance fields and the reduction of labor in maintenance.

The invention claimed is:
 1. A Mg-based alloy plated steel materialcharacterized by being provided with a hot dip Mg-based alloy platinglayer containing Zn: 15 atm % or more, Mg: over 35 atm %, and furthercontaining Ca and optionally one or more elements selected from thegroup of elements B: Al, and Y in a total of 0.03 to 15 atm %, whereinsaid hot dip Mg-based alloy plating layer comprises only crystal phase,and contains an intermetallic compound Zn₃Mg₇, wherein an X-rayintensity ratio of diffraction peak intensity of Zn₃Mg₇ relative to thesum of all diffraction peak intensities appearing at diffraction planespacing of 0.1089 to 1.766 nm is 10% or more, wherein diffraction peakof diffraction plane spacing of 0.233 nm is excluded from determiningsaid X-ray intensity ratio.
 2. An Mg-based alloy plated steel materialas set forth in claim 1, characterized in that said hot dip Mg-basedalloy plating layer containing Mg: 85 atm % or less.
 3. The Mg-basedalloy plated steel material as set forth in claim 1, characterized inthat said hot dip Mg-based alloy plating layer contains Mg: 55 to 80 atm%.
 4. The Mg-based alloy plated steel material as set forth in claim 1characterized in that said hot dip Mg-based alloy plating layer furthercontains one or more elements selected from the group of elements A: Si,Ti, Cr, Cu, Fe, Ni, Zr, Nb, Mo, and Ag in a total of 0.03 to 5 atm %. 5.The Mg-based alloy plated steel material as set forth in claim 1,characterized in that said hot dip Mg-based alloy plating layercontaining Zn: 20 atm % or more, Mg: 50 atm % to 75 atm %, and furthercontaining Ca and optionally one or more elements selected from thegroup of elements B: Al, and Y in a total of 0.03 to 12 atm %, here whensaid total is 1 to 12 atm %, containing Al: 1 atm % or more.
 6. TheMg-based alloy plated steel material as set forth in claim 5,characterized in that said hot dip Mg-based alloy plating layer is asingle Zn₃Mg₇ phase.
 7. The Mg-based alloy plated steel material as setforth in claim 1, characterized in that said hot dip Mg-based alloyplating layer is subject to holding said plating layer at a temperatureof a melting point of the Mg-based alloy plating to melting point ofMg-based alloy plating +100° C. for 1 minute or less, then rapidlycooling it.
 8. The Mg-based alloy plated steel material as set forth inclaim 7, characterized in that said rapid cooling is water cooling ormist water cooling.
 9. The Mg-based alloy plated steel material as setforth in claim 1, characterized in that the interface between said hotdip Mg-based alloy plating layer and steel material is provided with apreplating layer comprised of one or more elements selected from Ni, Cu,Sn, Cr, Co, and Ag.
 10. The Mg-based alloy plated steel material as setforth in claim 1, characterized in that said hot dip Mg-based alloyplating layer contains a balance of Mg and unavoidable impurities.